Dear T,
Yaaaaaaaaaaaaayyyyyy! Is it weird that I want to call or text you and say that? Of course I can't (or shouldn't). I'm sure I could email, but I probably won't. But it feels like we've gone through all this together. And even though you're my therapist and not my friend or partner or relative, it's like I want to celebrate with you.
Love,
LT

ETA: OK, sent a really short email, but I think that's OK. I mean...this is a really important moment.